Many types of dc-to-dc converters are known in the art for converting a first dc voltage to a second, regulated dc voltage. Typically, the dc input voltage is converted to an ac voltage (or dc pulses) by a switching transistor or transistors. The ac voltage is then converted to a regulated dc output voltage. Feedback of the output voltage may be used to control the duty cycle or the frequency of the ac voltage to achieve the desired voltage regulation.
Switching converters are known to have a higher efficiency than other types of dc power supplies, such as series-regulated power supplies. However, the efficiency of switching converters is limited by losses in the switching transistor(s) during turn-on and turn-off, particularly in pulse-width modulated (PWM) converters. In addition, the switching transistor(s) must simultaneously withstand high current and high voltage during both turn-on and turn-off.
Class E, Quasi-Resonant, and Bridge type resonant converters have all been used to achieve high frequency lossless switching. These circuits use high frequency inductors and capacitors to resonate the current or voltage across a device to zero in order to achieve low loss switching. These passive components cannot be integrated and therefore are not desirable for a very high density system.
Resonant converters use a variable frequency ac voltage for regulating the dc output voltage. Commonly assigned U.S. Pat. No. 4,672,528 of Park et al., which is incorporated herein by reference, describes such a resonant converter and provides a detailed background of the advantages and disadvantages of resonant converters. In resonant converters, it is possible to have either lossless turn-on or lossless turn-off, but not both. Furthermore, current in the transistor(s) of a resonant converter is relatively high. Because of these large currents, such resonant converters require costly transistors with high current ratings. Therefore, one of the primary goals of converter design is to reduce the transistor losses which degrade circuit efficiency and increase the cost of the converter.
Another goal of converter design is to reduce the size and weight. One proposed method of reducing the size and weight of the converter, while beneficially increasing the response time, is to increase the converter switching frequency. By increasing the switching frequency, a converter having smaller size, low weight, and faster response times can be obtained. The size and weight are decreased because the passive components required for operation at high frequency are smaller. However, the higher frequency switching aggravates transistor losses and degrades efficiency.
Normally, the switching devices utilized in switching power supplies are bipolar transistors, thyristors or field effect transistors. Although these devices may be modeled as ideal switches, it is well known that a more accurate model includes the parasitic effects of the device geometry. These parasitic components include diodes, capacitors and inductors whose effect on circuit operation may be minimized or ignored by proper design of the switching devices. Conversely, again by proper device selection or design, certain parasitic effects may be enhanced and beneficially employed in the operation of the circuit. Physical transformers also include nonideal parasitic elements which may be beneficially employed by proper design of the transformer and the switching circuit.
In order to reduce the expense, size and weight of conventional switching converters, it would be advantageous to design a switching converter which could utilize the parasitic characteristics of the switching devices and the isolation transformer of the switching circuit. Utilizing the parasitic characteristics of the switching devices and the transformer, it is possible to eliminate many of the discrete components of a switching converter which contribute substantially to its size, weight and cost.